GSK3β (glycogen synthase kinase 3β) is a proline directed serine, threonine kinase that plays an important role in the control of metabolism, differentiation and survival. It was initially identified as an enzyme able to phosphorylate and hence inhibit glycogen synthase. It was later recognised that GSK3β was identical to tau protein kinase 1 (TPK1), an enzyme that phosphorylates tau protein in epitopes that are also found to be hyperphosphorylated in Alzheimer's disease and in several tauopathies.
Interestingly, protein kinase B (AKT) phosphorylation of GSK3β results in a loss of its kinase activity, and it has been hypothesised that this inhibition may mediate some of the effects of neurotrophic factors. Moreover, phosphorylation by GSK3, of β-catenin, a protein involved in cell survival, results in its degradation by an ubiquitinilation dependent proteasome pathway.
Thus, it appears that inhibition of GSK3β activity may result in neurotrophic activity. Indeed there is evidence that lithium, an non-competitive inhibitor of GSK3β, enhances neuritogenesis in some models and also increases neuronal survival, through the induction of survival factors such as Bcl-2 and the inhibition of the expression of proapoptotic factors such as P53 and Bax.
Recent studies have demonstrated that β-amyloid increases the GSK3β activity and tau protein phosphorylation. Moreover, this hyperphosphorylation as well as the neurotoxic effects of β-amyloid are blocked by lithium chloride and by a GSK3β antisense mRNA. These observations strongly suggest that GSK3β may be the link between the two major pathological processes in Alzheimer's disease: abnormal APP (Amyloid Precursor Protein) processing and tau protein hyperphosphorylation.
Although tau hyperphosphorylation results in a destabilisation of the neuronal cytoskeleton, the pathological consequences of abnormal GSK3β activity are, most likely, not only due to a pathological phosphorylation of tau protein because, as mentioned above, an excessive activity of this kinase may affect survival through the modulation of the expression of apoptotic and antiapoptotic factors. Moreover, it has been shown that β-amyloid-induced increase in GSK3β activity results in the phosphorylation and, hence the inhibition of pyruvate dehydrogenase, a pivotal enzyme in energy production and acetylcholine synthesis.
Cdk5/p25, also known as tau protein kinase 2 (TPK2), is a proline directed, Ser/Thr kinase essential for central nervous system development and in particular for neuronal migration and neurite outgrowth. Cdk5 is a homologue of cyclin-dependent kinases and rather ubiquitously expressed. Its activator p35 (a 305 aa protein) or a truncated form p25 (208 aa, missing an N-terminal proline-rich domain not required for activity) are selectively expressed in neurons, limiting cdk5 kinase activity essentially to the CNS. Cdk5 is completely inactive in the absence of p35 or p25. The term cdk5/p25 will be used here for the active enzyme since evidence exists suggesting that p25 and less so p35 may be involved in pathological processes.
Physiological substrates of cdk5/p25 include DARPP-32, Munc-18, PAK1, synapsin 1 and perhaps some others. In addition, it is now well established that cdk5/p25 phosphorylates tau protein epitopes which are hyperphosphorylated in Alzheimer's disease. More recently, elevated cdk5/p25 activity, mislocalization of cdk5 and an increase in p25 activator has been found in the brain of Alzheimer patients. Interestingly, prephosphorylation of tau protein by cdk5/p25 considerably enhances phosphorylation of tau by GSK3β on other epitopes, also found hyperphosphorylated in Alzheimer's disease. Moreover, neurofibrillary tangles, the hallmark of Alzheimer's disease, are labelled with antisera for GSK3β and cdk5, but not GSK3α and MAP kinase, also, GSK3β and cdk5 are associated with microtubules and both, more than PKA and CK, contribute to the AD-like phosphorylation of tau protein. These results taken together suggest that mixed inhibitors of GSK3β and cdk5/p25 should efficient in protecting tau protein from hyperphosphorylation. Therefore, they would be useful in the treatment of any pathological disorder associated with the abnormal phosphorylaton of tau protein, in particular Alzheimer's disease, but also other tauopathies (e.g. frontotemporoparietal dementia, corticobasal degeneration, Pick's disease, progressive supranuclear palsy).
Cdk5/p25 has been linked to apoptosis and neurodegeneration in more general terms. Its overexpression induces apoptosis in cultured neurons, in brain tissue apoptotic cells show strong immunoreactivity for cdk5. Neurotoxic agents, incl. Aβ(1-42), neuronal injury, ischemia or growth factor withdrawal lead to activation and mislocalization of cdk5/p25, abnormal phosphorylation of cdk5 substrates, cytoskeletal disruption and cell death. Moreover, phosphorylation by cdk5/p25 transforms DARPP-32 into an inhibitor of protein kinase A, reducing signal transduction in the striatum with obvious implications for Parkinson's disease. A role for cdk5 in ALS has also been proposed based on its ability to phosphorylate neurofilaments. More recently, deregulation of cdk5 was detected in a mouse model of amyotrophic lateral sclerosis.
Altogether, these experimental observations indicate that GSK3β inhibitors may find application in the treatment of the neuropathological consequences and the cognitive and attention deficits associated with Alzheimer's disease, as well as other acute and chronic neurodegenerative diseases. These include, in a non-limiting manner, Parkinson's disease, tauopathies (e.g. frontotemporoparietal dementia, corticobasal degeneration, Pick's disease, progressive supranuclear palsy) and other dementia including vascular dementia; acute stroke and others traumatic injuries; cerebrovascular accidents (e.g. age related macular degeneration); brain and spinal cord trauma; peripheral neuropathies; retinopathies and glaucoma.
In addition GSK3β inhibition may find application in the treatment of other diseases such as:
Non-insulin dependent diabetes (such as diabetes type II) and obesity; manic depressive illness; schizophrenia; alopecia; cancers such as breast cancer, non-small cell lung carcinoma, thyroid cancer, T or B-cell leukemia and several virus-induced tumors.
Since it appears that both, GSK3β and cdk5/p25 play a major role in the induction of apoptosis in neuronal cells, combined inhibition of these two enzymes may find application in the treatment of not only Alzheimer's disease and the other above-mentioned tauopathies, but also in a number of other neurodegenerative disorders, in particular Parkinson's disease and amyotrophic lateral sclerosis; other dementias including vascular dementia; acute stroke and other traumatic injuries; cerebrovascular accidents (e.g. age related macular degeneration); brain and spinal cord trauma; peripheral neuropathies; retinopathies and glaucoma.
In addition mixed TPK1/TPK2 inhibitors may find their applications in the treatment of other diseases such as: smoking cessation and other withdrawal syndromes, epilepsy.